The invention relates to a device for control of the core of a nuclear reactor by means of clusters with diverse functions. such as control clusters and spectral shift clusters.
A method which provides for better utilization of nuclear fuel and reduced uranium fuel costs involves shifting the neutron energy spectrum in the core of the reactor from "soft" to "hard" during the early phase of the operating cycle or core life. For this purpose, it has been proposed, for example, during the first part of the reactor operating cycle, to introduce into the core the rod clusters of material which preferentially absorbs slow neutrons (typically fertile material which can be converted into fissile material under the action of the slow neutrons, such as depleted uranium). A hardening of the neutron spectrum is thus produced both by the reduction in the volume of the moderator in the core and by absorption of low-energy neutrons. In a second part of the reactor operating cycle, the rod clusters which exhibit non-fission slow neutron absorption are withdrawn from the reactor core and the fissile material in the nuclear fuel contained in the assemblies during the first phase is consumed. Reference may be made, for example, to European Patent Application Nos. 108,019 and 108,020.
In such reactors, joint use is then made of clusters of neutron-absorbing rods, which may be inserted to a greater or lesser extent into the assemblies forming the reactor core to control the latter, and of spectral shift clusters which are completely inserted into the fuel assemblies forming the reactor core during the first part of the operating cycle. In particular, the control clusters and the spectral shift clusters may be associated with the same reactor core assemblies, with a common motorized drive.
European Patent Application No. 111,435 describes a device incorporating control clusters and spectral shift clusters which are coaxial, each with a rotational symmetry of distribution of the rods in the fuel assembly with which they are associated. Movement of the clusters is produced by two coaxial control shafts rectilinearly moveable along a fluid-tight enclosure and equipped with means for locking the shafts in a plurality of positions. Each shaft connected to a control cluster is actuated by conventional means comprising electromagnetic coils and pawls. On the other hand, each shaft for controlling a spectral shift cluster, which is mounted coaxially inside the shaft of the control cluster, comprises a piston slidably accomodated in the control shaft and upwardly moved upon opening of a valve for pressure/release in the upper part of the fluid tight enclosure along which the control shafts are movable. In its uppermost position, the spectral shift shaft can be coupled to the control cluster shaft by fingers which engage in a groove in the spectral shift shaft.
While such a fine-control device is generally satisfactory, it nevertheless has the disadvantage that the motions of the shafts are not entirely independent, since the spectral shift shaft is subject to the motions of the control shaft. The spectral shift clusters are consequently partly inserted even when insertion is not desirable.
French Pat. No. 2,168,564 discloses fuel assemblies associated with guide/tubes along which a plurality of cluster control shafts slide individually. The rods of these clusters are distributed with a rotational symmetry in relation to the rod distribution pattern in the fuel assembly. While such a device provides complete uncoupling in guidance and movement of the various control shafts, as well as a uniform distribution of the rods in the fuel assembly, it has the disadvantage of being very bulky due in particular to the requirement for control and guidance means situated inside and outside the vessel and associated with each of the control shafts. For a description of hydraulic and electromechanical drive means for such shafts, reference may be had to French Pat. No. 2,232,820.